refactor(i18n): key messages by English text; add Russian

Replace the semantic keys with the English source text, so code reads as the
UI does and English needs no catalog: t('Save keys'), tn(count, one, other)
for plurals, msg() for text stored in data, and bare data-i18n* markup that
translates the element's own English. Other languages are one flat JSON each.

- Simplify src/i18n: drop namespaces, the configurable locale pair
  (GEV_DEFAULT_LOCALE / GEV_SECONDARY_LOCALE) and its build defines. Locale
  comes from ?lang=, the stored choice, then browser languages.
- Add scripts/i18n.mjs (npm run i18n): extracts every message from t/tn/msg
  and markup, and fails on missing, unused, placeholder-mismatched or
  wrongly-shaped plural entries. npm test runs it via catalog.test.mjs.
- Add Russian (src/i18n/locales/ru.json, 833 messages), started from the
  community catalog in #312 and reviewed against the current English.
- Fix data-i18n placed on parents that would wipe their children (cockpit
  LEVEL and local weather cells, first-run EXPLORE MANUALLY tile), and a
  doubled degree sign in the TRACK ACQUIRED signal.
- Rewrite docs/TRANSLATORS.md for the new workflow; drop sections the earlier
  merge resurrected in docs/CURRENT-STATE.md.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/data/vesselLabels.js

@@ -4,7 +4,7 @@
  * after the vessel cards moved out of their dedicated canvas renderer.
  */
 
-import { t } from '../i18n/index.js';
+import { t, msg } from '../i18n/index.js';
 
 export const VESSEL_OVERLAY_SOURCE_ID = 'ais-live-vessels';
 /** Existing selector grid size; one ambient winner is retained per cell. */
@@ -70,25 +70,25 @@ const NUMERIC_TYPE_FAMILIES = {
 // machine values — TYPE_STYLES matches on them and tests pin them — so card
 // text translates only through this map at the presentation boundary.
 const TYPE_DISPLAY_KEYS = Object.freeze({
-  FISHING: 'layers.vessel.type.fishing',
-  TOWING: 'layers.vessel.type.towing',
-  DREDGER: 'layers.vessel.type.dredger',
-  'DIVE OPS': 'layers.vessel.type.diveOps',
-  MILITARY: 'layers.vessel.type.military',
-  SAILING: 'layers.vessel.type.sailing',
-  PLEASURE: 'layers.vessel.type.pleasure',
-  PILOT: 'layers.vessel.type.pilot',
-  SAR: 'layers.vessel.type.sar',
-  TUG: 'layers.vessel.type.tug',
-  'PORT TENDER': 'layers.vessel.type.portTender',
-  'ANTI-POLLUTION': 'layers.vessel.type.antiPollution',
-  'LAW ENFORCE': 'layers.vessel.type.lawEnforce',
-  MEDICAL: 'layers.vessel.type.medical',
-  'HIGH-SPEED': 'layers.vessel.type.highSpeed',
-  PASSENGER: 'layers.vessel.type.passenger',
-  CARGO: 'layers.vessel.type.cargo',
-  TANKER: 'layers.vessel.type.tanker',
-  OTHER: 'layers.vessel.type.other',
+  FISHING: msg('FISHING'),
+  TOWING: msg('TOWING'),
+  DREDGER: msg('DREDGER'),
+  'DIVE OPS': msg('DIVE OPS'),
+  MILITARY: msg('MILITARY'),
+  SAILING: msg('SAILING'),
+  PLEASURE: msg('PLEASURE'),
+  PILOT: msg('PILOT'),
+  SAR: msg('SAR'),
+  TUG: msg('TUG'),
+  'PORT TENDER': msg('PORT TENDER'),
+  'ANTI-POLLUTION': msg('ANTI-POLLUTION'),
+  'LAW ENFORCE': msg('LAW ENFORCE'),
+  MEDICAL: msg('MEDICAL'),
+  'HIGH-SPEED': msg('HIGH-SPEED'),
+  PASSENGER: msg('PASSENGER'),
+  CARGO: msg('CARGO'),
+  TANKER: msg('TANKER'),
+  OTHER: msg('OTHER'),
 });
 
 /**